1. FIELD OF THE INVENTION
The invention relates to regeneration of spent catalyst from an FCC unit in a way which reduces NOx emissions.
2. DESCRIPTION OF RELATED ART
NO.sub.x or oxides of nitrogen in flue gas streams from FCC regenerators is a pervasive problem. FCC units process heavy feeds containing nitrogen compounds and much of this material is eventually converted into NOx emissions, either in the FCC regenerator (if operated in full CO burn mode) or in a downstream CO boiler (if operated in partial CO burn mode). Thus, all FCC processing nitrogen containing feeds can have a NOx emissions problem due to catalyst regeneration, but the type of regeneration employed (full or partial CO burn mode) will determine whether NOx emissions appear sooner (regenerator flue gas) or later (CO boiler).
Although there may be some nitrogen fixation or conversion of nitrogen in regenerator air to NOx most of the NOx emissions are believed to come from oxidation of nitrogen compounds in the feed.
Several powerful ways have been developed to deal with the problem. The approaches fall into roughly five categories:
1. Feed hydrotreating to keep NOx precursors from the FCC unit.
2. Segregated cracking of fresh feed.
3. Process approaches which reduce the NO.sub.x formation in a regenerator in complete CO burn mode via regenerator modifications.
4. Catalytic approaches, using a catalyst or additive which is compatible with the FCC reactor, which suppress NOx formation or catalyze its reduction in a regenerator in complete CO burn mode or reduction of NO.sub.x precursors (e.g. NH.sub.3) to the CO boilers in partial CO burn mode.
5. Stack gas cleanup methods which are isolated from the FCC process.
The FCC process will be briefly reviewed, followed by a review of the state of the art in reducing NO.sub.x emissions.